Musically, "Lights Up" is a pop and R&B song, featuring multilayered guitars, piano, programmed beats, and a gospel choir.

Critics found Styles's musical direction refreshing and commented on the song's unconventional structure which is composed of several breakdown pre-choruses and post-choruses, and a single chorus.

Vincent Haycock directed the song's music video, which features Styles dancing shirtless in a sweat-drenched crowd of people.

Formerly a member of the boy band One Direction, Harry Styles emerged as a solo artist in 2017 with his self-titled debut studio album, which heavily incorporated a 1970s rock-influenced sound.

[4] Styles wrote the song with Johnson and Harpoon,[13] who is credited under his birth name Thomas Hull in the liner notes.

[18] Critic Jon Caramanica of The New York Times characterised its sound as "somewhere between '70s soft rock, lite disco and indie pop".

[23] Slate writer Chris Molanphy described the song as "lightly strummed beach music with ethereal backing vocals".

[1] Now writer Rea McNamara said that the song was "an all-too-brief ode to self-love and letting go",[34] and in the words of Vulture critic Craig Jenkins, it is about "piercing the darkness in our hearts with radiant light".

[10] Tallahassee Democrat's Jia Alonso and The Washington Post's Anying Guo associated the song's lyrics to be vaguely about Styles's sexuality.

[32][39] The billboards also bore the acronym "TPWK" (Treat People with Kindness), a phrase that had earlier been used on the merchandise of Harry Styles: Live on Tour.

[49][50][51] Hilary Hughes of Billboard praised the performance, writing: "With little more than the piano, an acoustic guitar, a trumpet and the intricate harmonies of his back-up singers, Styles belted every high note and danced to the beat of the song's bridge".

[49] Styles performed the track again on Later... with Jools Holland on 21 November and at Capital FM's Jingle Bell Ball on 7 December.

[52][53] The song was included on the setlist of his one-night concert at the Forum in Los Angeles on 13 December to promote the release of Fine Line.

[19] The Atlantic's Spencer Koornhaber said the track rendered the type of eerie yet simple listening territory that had seldom been explored since Donovan's "Mellow Yellow".

[23] To explain this viewpoint, she highlighted that Styles's identity often felt lost in the middle of musical tropes on his debut album.

According to Pitchfork writer Anna Gaca, the track was "designed to wriggle through the strictures of pop songwriting",[27] and Chris DeVille from Stereogum claimed that it showed Styles's ability to "toy around with accessible sounds without descending into the blasé".

[58] The detailed arrangement of horns, congas, and choir on the song was noted approvingly by Clash editor Susan Hansen, who selected "Lights Up" as the best track on Fine Line and praised its "subtle, but acute build up".

[22] Variety's Chris Willman picked it as one of the "distinct modern outliers" on the album, alongside "Adore You" and "Watermelon Sugar".

[59] Andrew Unterberger was more critical in his review for Billboard in which he singled out the song's direction as deceptive and wrote that the track "never quite tells you where it's going and then leaves you off somewhere you don't even recognise".

[60] Tim Sendra of AllMusic called the single "inoffensive and sweet", saying it was "only saved from the skip button by the always impressive vocals".

[70] In the United States, "Lights Up" debuted and peaked at number 17 on the Billboard Hot 100 chart dated 26 October 2019, accumulating 21.5 million streams and 20,000 download sales in its opening week.

[72] The Recording Industry Association of America (RIAA) certified the song double Platinum for track-equivalent sales of two million units.

[33][102] Alonso noted swift transitions between every scene in the music video which has Styles as the central point of every location change.